1. Field of the Invention
The present invention relates to a globular glass manufacturing apparatus and a method for manufacturing the globular glass, for manufacturing a globular glass used for a preform, lens and the like to obtain an optical element by precision mold pressing, such as a lens and prism used as a light pickup lens for DVD, CD and magnet-optical disk (MO), lens for a cell-phone camera, lens for light communication and optical devices, and the like.
2. Description of Related Art
In recent years, in order to reduce manufacturing cost, optical elements such as lens are manufactured with precision mold pressing that a glass preform is heated to be soft and press-molded with a metal die having precise molding surface to obtain optical element such as lens directly in which grinding and polishing steps following to the molding are omitted. A micro globular preform is becoming to be used in manufacturing a micro lens such as, for example, a light pickup lens for such as DVD, lens for a cell-phone camera and optical coupling lens used in optical communication field to couple a photo diode or laser diode with a fiber optics with precision mold pressing. Furthermore, a micro ball lens is sometimes used as the above light coupling lens and the like. Thus, a globular glass having diameter of about 0.8 mm to 2.4 mm is required as the above micro preform, micro globular lens or the like.
It is a general method for mass-producing the preform at low cost that let glass melted in a glass fusion furnace flow from a flow nozzle and the flowing glass is divided and cooled by some method. Here, the following method is known for dividing a glass flowing from the tip of the flow nozzle to manufacture a glass gob having small diameter. That is, let the molten glass melted in the fusion furnace flow from the flow nozzle in a form of continuous flow, and let the continuous flowing molten glass deform to droplet-like glass gobs falling in a line by the action of the surface tension. Then the glass gobs falling in a line is caught into a liquid in a tank to collect the glass gobs used as a preform, lens and the like (for example, see JP Tokukaihei 09-235122A).
In manufacturing a glass gob disclosed in the above JP Tokukaihei 09-235122A, the molten glass flowing in a form of continuous flow is deformed into the droplet-like glass gobs by cutting the molten glass with the surface tension during fall. However, it has been problematic in this process that the two glass gobs neighboring above and below sometimes contact or collide with each other and re-couple to be one glass gobs again, since the two glass gobs pulls each other by the action of the surface tensions so that there occurs a difference in falling speed between the two glass gobs during fall in a line.
Such re-coupled glass gob has various shape according to a temperature and viscosity in the coupling, such that the two glass gobs couple with each other to be one entirely, couples with each other partly, and the like. The obtained glass gobs have various shape are nonuniform in their weight and shape.
Further, even if they re-couple to be one, it sometime has striae internally. Therefore, it is impossible to use it as an optical element.
Further, even when the glass gobs neighboring above and below do not re-couple but contact each other, they sometimes has a flaw on their surface. Thus it is impossible to use them as an optical element.
Therefore, it is required that the glass gobs have to be collected before the glass gobs neighboring above and below re-couple or contact each other. This problem is possibly avoided by shortening falling distance of the glass gobs. However, when the falling distance is short, it is requested to collect the glass gobs having high temperature. Thus, the glass gobs have to be caught into oil or the like at high temperature to collect them in order to avoid generation of a crack caused by thermal shock. Concretely, when the glass gobs having high temperature and low viscosity are caught into oil or the like at high temperature, it s problematic that the obtained glass gob has nonuniform shape due to deformation caused by impact of collision with liquid surface. Further, when the glass gob is caught into oil at high temperature to collect it, the glass gob requires careful wash to use it as a preform or optical element. Thus, it is problematic that the wash takes time and cost and the glass gob is easily damaged in the wash. Further, there is a problem on safety ground that the oil may take fire when the glass gob at high temperature is caught into oil at high temperature.
In the case that the glass gob is used as a preform for precision mold pressing, the preform having nonuniform weight and a internal striae caused by the above-described re-coupling and the like as described above cannot be used as an optical element even if the preform is molded with precision mold pressing. When a preform having nonuniform shape is molded with precision mold pressing, a molded product sometime has a defect that the surface thereof partially cave in (sink mark) and the optical axis deviates because of the uneven thickness of the product. Thus, it sometimes cannot be used as an optical element. When a preform having a flaw is molded with precision mold pressing, defects sometimes occurs, such that the flaw itself or the glass surface around the flaw is fold to be incorporated into the inside of the glass and the glass solidifies as it is (folded ridges) and gas is incorporated into the inside thereof from the flaw to be a micro bubble inside the product. Thus, the product sometimes cannot be used as an optical element.
On the other hand, a method for manufacturing a ball glass is disclosed, in which molten glass kept in a container is ejected to gas phase from an orifice provided to a part of the container so as to form a glass droplet, while vibration is given to the molten glass, and the glass droplet is solidified while the glass droplet is falling in gas phase or liquid phase (for example, see JP Tokukai 2003-104744A).
In the method for manufacturing a ball glass disclosed in the above patent application, vibration is given to the molten glass. A concave portion corresponding to the vibration is formed on a side of a columnar glass droplet which continues from an outlet to decrease the sectional diameter of the columnar glass droplet. According to increase of the falling speed, the columnar glass droplet in which a concave has been formed is separated into droplets having controlled volume at the concave portion formed by the vibration. However, when the columnar glass droplet is separated at the concave portion, there occurs a difference in falling velocity between the separated two glass gobs neighboring above and below caused by the similar reason to the case of the above JP Tokukaihei 09-235122A. When some force (JP Tokukai 2003-104744A does not disclose anything related to the force, but for example, irregular disturbance such as a gas phase convection generated in a collection chamber can be considered) is given to the separated glass droplets so that they have various falling trajectory one another as shown in FIG. 1 of JP Tokukai 2003-104744A, it is comparatively less possible to occur re-coupling of the two glass droplets neighboring above and below and generation of a flaw caused by a contact of the two glass droplets neighboring above and below. However, when such force is not given or is merely given to the falling droplet, the separated glass droplets falls in a line or almost in a line. Thus it is possible to occur re-coupling of the two glass droplets neighboring above and below into one and generation of a flaw caused by a contact of the two glass droplets neighboring above and below caused by the similar reason to the case of the above JP Tokukaihei 09-235122A. In order to avoid these problems, the glass droplets have to be caught into oil at high temperature, which has the similar problem to the method for manufacturing a glass gob disclosed in JP Tokukaihei 09-235122A.
The above JP Tokukai 2003-104744A discloses that the columnar glass droplet is separated to glass droplets of constant volume according to the vibration rate so that the separated glass droplets having uniform volume can be obtained. When a ball glass having diameter larger than the inner diameter of the outlet of the orifice provided to a part of the container is desired, if desired diameter of the glass is slightly larger than the inner diameter of the outlet, it can be obtained by letting a glass droplet have its diameter slightly larger than the inner diameter of the outlet by the action of the surface tension. However, in order to obtain a ball glass having further larger diameter, the container have to be changed to that having larger outlet inner diameter of the orifice, and let glass droplets having larger diameter flow. Therefore, when various ball glasses having considerably different volumes each other are manufactured, several kind of containers having various outlet inner diameter of the orifice have to be prepared. The container which keeps molten optical glass is generally made of platinum in order to obtain an optical glass of high quality. Since such platinum made container is extremely expensive, it is a big economical disadvantage to prepare various kinds of the containers. Further, it is also problematic that an exchange of the container costs labor and time.
According to the technique disclosed in JP Tokukai 2003-104744A, molten glass is ejected from an orifice provided to a part of a container to gas phase so as to form a glass droplet. Thus, once ejection of the molten glass starts, it is difficult to stop the ejection of the molten glass unless the entire molten glass in the container is ejected. Therefore, there is a problem on safety ground since it is difficult to bring the ejection of the molten glass to emergency stop.